The rapid advances in the development of larger and faster semiconductor power switches, such as the gate turn-off thyristor, are resulting in increasing use of electronic systems in the processing of ac and dc power, in new applications and with new performance requirements which previously were impractical. Examples of this include the interfacing of dc alternative energy sources (such as fuel cells or solar arrays) to the ac line, the supply of dc power in Navy shipboard applications, conditioning of variable frequency power from wind generators for the ac line, the variable speed control of ac motors, the dc transmission of electrical power, and the construction of uninterruptible power supplies.
At higher power levels, three-phase ac systems are invariably used, and hence at least one port of any high-power switched-mode converter must interface a three-phase ac system. So far, the most exploited application area of three-phase ac converters is the variable speed control of ac motors. The most objectionable feature of converters presently used in these drives is the generation of output frequency harmonics. These harmonics lead to lower power factors, higher losses, distorted line voltage waveforms, pulsation torques in induction motors, and radiated electromagnetic interference. Another problem associated with three-phase ac converters is their inability to provide variable voltages above the dc input value.
Attempts to develop converters with more accurate control of converter voltage and current waveforms have resulted in the discovery of a number of dc to three-phase switched mode converters. Ngo, et al, in an article entitled "A New Flyback DC-To-Three-Phase Converter With Sinusoidal Outputs", IEEE Power Electronics Specialists Conference, 1983 Record, pp 377-388, describe a poly-phase converter which employs an energy transfer inductor and seven transistor/diode pairs to generate a sinusoidal output. However, since the converter produces pulsating output currents, a capacitor filter network is required to obtain a "clean" output.
The problems of pulsating input and output currents are known in the art. Various dc to dc converter designs have been proposed to avoid such a phenomenon. In U.S. Pat. No. 4,184,197 to Cuk, at al, a dc to dc switching converter is described which employs a pair of inductances, one in series with the input source and the other in series with the output load. An energy transfer device (capacitor) is switchably controlled to transfer the input energy to the output. That circuit, as well as others, are further described by Cuk, et al, in "A New Optimum Topology Switching DC-to-DC Converter" appearing in "Advances in Switched Mode Power Conversion", Middlebrook & Cuk, Vol 2, 1983, pp 311-330. Other dc-to-dc converter designs which avoid the pulsating terminal current phenomenon are described by Erickson in "Synthesis of Switched-Mode Converters", Power Electronics Specialists Conference, 1983 Record, pp. 9-22. While non-pulsating currents have been obtained in dc to dc converters, it has not been heretofore known how to efficiently obtain this property in a three-phase converter.
Accordingly, it is an object of this invention to provide a class of three-phase, switched-mode converters which exhibit non-pulsating terminal currents.
It is a further object of this invention to provide three-phase switched-mode converters which employ minimal component counts in order to achieve the above-mentioned characteristics.
Still another object of this invention is to provide a variable frequency, three-phase switched-mode converter which is adapted to generate output voltages both above and below the dc input value.